Switching converters are usually operated in different operating modes dependent on one or more parameters such as the input voltage, the output voltage and the output current (i.e. the load). The different operating modes differ in the switching control of the electronic switches that control the current flow through an inductor (choke) of the switching converter. Switching converters can be operated in continuous conduction mode (CCM) and discontinuous conduction mode (DCM). In CCM the inductor current continuously passes through the inductor without dropping to zero, whereas in DCM, the inductor current has a discontinuous waveform as the current drops to zero in each switching cycle.
The switching operation of electronic switches is determined by a switching signal that triggers activation/deactivation of the electronic switches. The switching signal is a logic signal (i.e. assumes only a logic high level or a logic low level) than can be modulated using different modulation schemes in order to regulate, e.g., the output voltage or the output current. Common modulation schemes are pulse width modulation (PWM) and pulse frequency modulation (PFM). When using PWM, the duty cycle of the switching signal is adjusted by adjusting the on-time (i.e. the pulse width) of the switching signal in each switching cycle, while the switching frequency is constant. When using PFM, the switching frequency is adjusted by adjusting the temporal position of the pulses of the switching signal, while the pulse-width (i.e. the on-time in a switching cycle) is constant.
Both modes, CCM and DCM, can be combined with one of the modulation schemes PWM and PFM. In this example four operating modes can be distinguished, i.e. PFM-DCM, PWM-DCM, PWM-CCM, and PFM-CCM. The achievable efficiency of the power conversion can be different for different operating modes for a specific operating point (e.g., for a specific combination of input voltage, output voltage, output current). Further, some operation modes may not be suitable for a specific range of operating modes. In order to optimize efficiency, multimode switching converters have been developed, which are configured to operate in two or more different operating modes. The operating point of the switching converter is monitored and, if a mode switch condition is reached, the operating mode is changed.
In some operating modes stability problems may occur, whereas a stable operation is usually ensured when using PWM-CCM. However, particularly PFM-CCM may cause undesired behavior of the switching controller in a specific range of operating points such as operating points with high input voltage (as compared to the output voltage) and high output current (high load).